The present invention relates to a method of manufacturing a dye-sensitized solar cell (DSSC) and the solar cell, and more particularly, to a dye-sensitized solar cell having a means capable of increasing light absorption to thus improve efficiency of the cell and a method of manufacturing the same.
A dye-sensitized solar cell (DSSC) is a solar cell that was invented at early 1990's using a photosynthesis principle of a plant by Professor Gratzel of Swiss Federal Institute of Technology Zürich and is a sandwich-type cell in which an electrolyte solution containing oxidation/reduction bands is interposed between two glass substrates having a transparent conductive film coated thereon. After the DSSC is published, researches haven been performed at home and abroad, including universities, research institutes and manufacturers. In particularly, many efforts have been made so as to increase energy conversion efficiency thereof.
In an existing solar cell, an absorption process of solar energy and a process of separating an electron-hole pair to make a current of electricity simultaneously occur in a semiconductor material. However, in the DSSC, the two processes are separated, so that the solar energy is absorbed in a dye and the movement of charges is made in a metal oxide nano-particle layer such as TiO2.
Basically, the DSSC includes upper and lower transparent substrates (for example, glass substrates), conductive transparent electrodes formed on surfaces of the transparent substrates, a photosensitive dye capable of absorbing a visible ray to generate an electro-hole pair and an oxide semiconductor electrode transferring the generated electrons and consisting of titanium oxide nano-particles (for example, refer to Korean Patent Application Publication No. 10-2010-132127). As the conductive transparent electrode, ITO (Indium Tin Oxide), recently FTO (Fluorine-doped Tin Oxide) having favorable stability at high temperatures is used in many cases. The electrons excited in the dye resulting from the absorption of the visible ray are transferred to the titanium oxide particles, which are an n-type semiconductor, and then transferred to the FTO to which the titanium oxide particles are contacted. At this time, the dye is regenerated through an electrochemical oxidation-reduction reaction of “I−/I3−) contained in the liquid electrolyte, so that current is generated.
The dye is transitioned from a ground state to an excited state due to the absorption of the photon energy. The excited electrons is introduced into a conduction band of the TiO2 nano-particles and is then moved to the transparent electrode and to a counter electrode through an external circuit. The dye oxidized due to the electron transfer is supplied with electrons from the electrolyte and is thus reduced. For example, platinum coated on the counter electrode exhibits a catalyst operation of reducing “I3−”of the oxidation-reduction pair to “I−” and an operation of increasing reflection efficiency of sunlight having transmitted the cell.
Since the DSSC can be manufactured at lower cost than the single crystal solar cell, the amorphous solar cell and the compound semiconductor solar cell, it attracts many attentions as next-generation solar cells. The DSSC can be manufactured at low cost, does not have a harmful material of constitutional components thereof and does not cause a pollution upon waste, which is environment-friendly. Also, since the DSSC is transparent and can exhibit various colors, depending on dyes to be used, when it is attached on a window or outer wall of a building, it can express excellent aesthetic properties. Further, the unevenness of efficiency deviations is relatively small with respect to orientations and incident angles of the sunlight. Therefore, it is expected that the roles thereof are increased in the solar photovoltaic such as a building integrated photovoltaic system (BIPV), rather than a large-scaled generator, as compared to the silicon solar cell.
Like this, the DSSC has the relatively high competitive power as regards the unit cost of production and the applications thereof. However, as compared to the other solar cells based on inorganic materials, the efficiency thereof is still lower, which is a setback of the rapid commercialization. That is, after the DSSC has been invented, it attracts an attention as a potential low-cost photovoltaic device, so that the substantial developments have been made over 20 years. However, the efficiency of the DSSC is still lower than the inorganic photovoltaic cell. This is mainly caused due to the relatively lower short-circuit current density. Accordingly, it is necessary to increase the photo current so as to improve the efficiency of the DSSC.
The main constitutional element of the DSSC is the electrode layer consisting of TiO2 nano-particles. The electrode layer provides a surface on which the dye directly absorbing the light is adsorbed (a sye molecule support member) and serves as a charge movement path (an electron transfer medium) moving the electrons coming from the dye. The DSSC of the related art has a high charge collection ability, a high open-circuit voltage and a favorable fill-factor. However, the DSSC does not completely absorb all photons from the visible ray and near infrared ray regions. As a result, the DSSC has the lower short-circuit photo current density than the inorganic photovoltaic device. Therefore, the main factors for improving the efficiency of the DSSC are focused on the increase in the short-circuit current density of the DSSC.
In the meantime, due to the porosity of the TiO2 electrode, the oxidation-reduction electrolyte is impregnated into the structure thereof and can be closely contacted to the semiconductor material. The TiO2 network is a receptor of the electrons generated from the photo-excited dye molecules and provides a conductive path for a collecting electrode. An oxidation-reduction species in the electrolyte transports the holes from the oxidized dye to the counter electrode. The porous TiO2 electrode is typically manufactured by coating a paste including TiO2 nano-particles on the conductive glass with a doctor blade or screen printing and then sintering (heat treating) the same at 450 to 500° C. The heat treatment process is required so as to remove an organic additive including a binder included in the pate and to sinter the TiO2 nano-particles therebetween. Since the TiO2 nano-particle layer relates to both generation and movement of the electrons, which dominates the performance of the cell, a nano-structure of the layer have attracted many attentions and have been researched. For example, electrode having other shapes such as TiO2 nanotubes, nanotubes having nano-particles filled therein and nanorods may be also used.
In the meantime, a typical Ru-based dye that has been used in the existing DSSC has a low coefficient of absorption, so that a necessary thickness of the TiO2 electrode is about 10 μm. Regarding this, when the thickness of the electrode is made to be thin, an amount of the dye to be adsorbed is not sufficient, so that an overall absorbance becomes too low. To the contrary, when the thickness of the electrode is increased, the amount of the dye to be adsorbed is increased to increase the absorbance. However, a moving distance of the electrons is correspondingly lengthened, which brings about a contrary effect as regards the performance of the cell.
The information disclosed in the background of the invention is provided only for enhancement of (or better) understanding of the background of the invention, and should not be taken as an acknowledgment or any form of suggestion that this information forms a prior art that would already be known to a person skilled in the art.